Drop volume is an important parameter for many processes in which drops are expelled as part of an operation of a device or as part of a fabricating process. For example, for inkjet printers, drop volume is an important factor for evaluating ink jetting performance, which in turn can be impact overall performance of the printer. In particular, drop volume data can be critical for development activities such as the early stages of designing ejector, or jet, geometry, formulating new inks, and developing specific printhead drive waveforms. It is know to determine drop volume indirectly by measuring the total weight of tens of millions of drops, for example, the drops are ejected and received on a substrate with a known weight. The substrate with the received drops is then weighted on a balance and the weight of the drops is derived by subtracting the known weigh of the substrate. However, this method is extremely time-consuming and expensive. Use of this method can undesirably prolong the development cycle for new products and increase the cost of the development cycle.
Drop velocity calculation, frequency sweep, and a drop volume frequency sweep are often performed to evaluate ink ejecting performance. It is known to use stroboscopic imaging to generate optical images of expelled drops to measure drop velocity. For example, the stroboscopic imaging system produces high frequency, intensive, short pulsed flashes of light that illuminate drops in flight and produce optical images of the drops in flight. Determining drop volume requires the much slower and cumbersome weighing procedure described above. A drop volume frequency sweep using the drop volume procedure noted above (weighing tens of millions of drops) typically requires hours (and undesirably large amounts of ink) to complete. In contrast a drop velocity calculation, frequency sweep measures can typically be completed in several minutes.
Due to the length of time and the amount of ink required to complete a single drop volume frequency test, it is time consuming and costly, if not impossible, to acquire drop volume frequency sweep data in those cases, for example, selection of single jet design or waveform development, in which tens or hundreds of frequency sweeps are needed. Furthermore the weighting method described above only works for steady jetting conditions in which all drops have the same volumes. In practice, drop velocity as well as drop volume vary considerably in any drop burst pattern, which also is a key factor for assessing jetting performance. The above weighing procedure is unable to measure volume of individual drop in burst pattern because it is an average measurement. In addition, to use the above weighing procedure for a printing application, tens of milliliters of ink are typically required to conduct a single drop volume frequency sweep. However, each batch of experimental inks is typically made in similar volume, for example, hundreds of milliliters. A typical ink evaluation set includes tens of other tests beyond drop volume. Therefore, due to the small volumes of ink typically generated for experimental inks, drop volume determination at all desired conditions is often not possible.